Alternating current electric motors, and particularly a.c. induction motors, tend to waste a considerable amount of electric power when operating with anything less than a full load coupled with their output shaft. Induction motors are the mainstay of certain widespread applications: in particular, they are almost universally used in refrigerators and air-conditioners because they have many features which make them attractive for inclusion in the usual unitized "hermetic" motor/compressor assembly typical of such appliances. Such features include proven reliability, absence of brushes, simple and cheap construction, relatively quiet operation, and a good history of predictable design performance. Induction motors also find widespread application in other domestic and commercial appliances, such as washing machines, clothes dryers, dishwashers, pumps, compressors, and so forth. Again, their advantage is cheap, simple design and predictable performance.
Induction motors are particularly prone to ENERGY LOSS when operating with less than full load. A typical 1/3-horsepower induction motor, which might be typified by a General Electric model H35JN30T, draws about 6.6 amperes under full load and exhibits a power factor of about 80% or so. Under light load, and particularly under no-load, this same motor exhibits miserable actual efficiency because the internal losses remain high while the output power demanded from the motor lessens. Although the power factor may drop to 30% or so, the apparent current still remains high . . . on the order of 4.9 amperes. As such, even though the 607 watts draw under full load may drop to about 170 watts under no-load: it is the range of operation between full load and this later no-load (or lightly loaded) value which is the basis for considerable improvement in my invention. At half-load, the power draw remains high, being nearly 360 watts. The following BASIC computer routine may be used to determine not only running efficiency, but also wasted power:
__________________________________________________________________________ 10 REM MOTOR EFFICIENCY DETERMINATION MOTEFF-1.BAS V1.01 20 REM MBASIC-80 (c) H. Weber K1VTW 9/9/89 30 PRINT CHR$(27) + "[2J"+ CHR$(27) + "[f" ' clear screen and home cursor 40 PRINT "Enter A.C. LINE VOLTAGE: ";:INPUT LV 50 PRINT " Motor OUTPUT (Decimal H.P.) ";:INPUT HP 60 PRINT " APPARENT Motor CURRENT ";:INPUT MI 70 PRINT " ACTUAL POWER FACTOR ";:INPUT PF 80 EF = ((74600!/(MI*LV*PF))*HP)*100 90 PWX = (MI*LV*PF*(100-EF))/10 4 100 PRINT:PRINT "MOTOR EFFICIENCY is: "EF" percent" 110 PRINT "WASTED Motor POWER is: " PWX "watts" 120 PRINT:PRINT:END __________________________________________________________________________
Using this routine, you will obtain the following display when entering full-load and half-load values:
______________________________________ Enter A.C. LINE VOLTAGE: ? 115 Motor OUTPUT (Decimal H.P.) ? .333 APPARENT Motor CURRENT ? 6.6 ACTUAL POWER FACTOR (percent) ? 80 MOTOR EFFICIENCY is: 40.9121 percent WASTED Motor POWER is: 358.782 watts ______________________________________ Enter A.C. LINE VOLTAGE ? 115 Motor OUTPUT (Decimal H.P.) ? .167 APPARENT Motor CURRENT ? 5.8 ACTUAL POWER FACTOR (percent) ? 55 ______________________________________
It is well known that eddy current losses and winding losses contribute most of this power waste, particularly when operating under less than full load. This power waste appears as heat, producing "temperature rise" within the motor structure. Also known is that the apparent current (e.g., 5.8 amperes at half-load) must circulate through the winding, and the induced magnetic field must magnetize the core material of the stator. It is only that the energy stored in the inductance of the core "returns" energy to the system that some semblence of efficiency is obtained, observable as low power factor manifested as phase lagging current flow. Large power loss occurs because the apparent current flow must overcome all the possible "friction" losses of the core material and the winding resistance. In cheap commercial motors particularly, these losses can be substantial. Economy motors are designed to operate with high current density in their windings, and with near-saturation of the core material.
When an ordinary induction motor is lightly loaded, the rotor "speeds up" with the result that the stator inductance actually tends to increase, resulting in the low power factor intrinsic with unloaded or lightly loaded induction motor operation. Clearly it would be better if the motor's rotor did not speed-up, but instead that it would continue to slip or drag by about the same amount under light load as what it does under full load. By reducing the applied stator voltage, the field is weakened and the rotor torque is lessened resulting in this desirable condition of slip or drag. The benefit is that the current power factor remains high, nearly that obtained under full load with full power applied. Mere reduction of the applied stator voltage is, by itself, unacceptable in most motor applications because any unexpected increase in motor loading can result in stalling and unsatisfactory operating characteristics, and can even lead to motor burnout.
Modern high-permeability core materials may also exhibit a somewhat more abrupt "knee" where saturation occurs. With an economy design approach, wherein the operating point for the core material making up the motor's stator structure is established with a high flux density under normal line voltage, it can be seen that an unsual increase in line voltage can bring about a very serious decrease in efficiency as saturation of the core material is approached. Under such a condition, the increased line voltage contributes nothing except power waste to the overall operation of the motor. Such losses tend to be regenerative, in that the mentioned increase in losses produces more heating, which in turn increases the losses (i.e., winding resistance loss, etc.).
Electric utility companies frequently introduce "brown-out" conditions during peak-usage periods or during unseasonable load demand periods (such as most notably, during a hot and humid summer period when air-conditioners are working hard). In the ordinary motor construction, such a brown-out condition can cause failure of induction motors, with stalling and overheating. My invention might be useful in overcoming these brown-out attendant problems, at least in critical applications where the stoppage of a motor can not be afforded. For example, in this kind of "brown out resistant" configuration the motor may be designed to produce its full torque (e.g., horsepower) at a reduced voltage level of say 100 volts and the control system of my instant invention will allow the motor to still accomodate line voltage operating conditions of 117 or even 125 volts or more without undue electrical loss or malperformance.
Economy motor designs are not only found in motors like the mentioned major appliance motor, but also they are ubiquitously found in the motors used in hermetic sealed refrigeration and air conditioning motor/compressor units. Induction motors of ordinary split-phase or capacitor start design are known in hermetic units, such as a Whirlpool model S462544/H2269; General Electric model PS-36-1/4; Americold model ML090-1; Tecumseh model S4416; Matsushita model FN91F17R, and others.
In my prior U.S. Pat. No. 4,806,838 "A.C. Induction Motor Energy Conserving Power Control Method and Apparatus" and U.S. Pat. No. 4,823,067 "Energy Conserving Electric Induction Motor Control Method and Apparatus" I particularly teach how motor losses may be greatly reduced through the use of two separate parallel-acting RUN windings. One higher impedance RUN winding supplies a sufficient portion of the field strength flux to operate the motor under partial load, while the other lower impedance RUN winding is modulated with a.c. power to increase the field strength flux as the motor load increases. In the '838 patent, I sense the power factor of the motor and as the power factor decreases when the motor loading lessens, I reduce excitation to the modulated RUN winding thereby increasing the apparent power factor. In my other '067 patent I utilize load-related changes in sub-synchronous motor speed slip to establish corresponding changes in the modulated RUN winding excitation.
In both of these prior patents a unique motor winding arrangement is needed in order to obtain increased efficiency. It was not the purpose of these prior inventions to necessarily be applicable to pre-existing motors, such as found in refrigerators, air conditioners, and other appliances. It was more the intent for the invention of these prior patents to provide a convienent and effective arrangement for manufacturers to use in their new motor designs in order to obtain a major increase in efficiency.
Older motors may also benefit from the kind of a.c. power control taught under these prior patents, but in order to do so a motor controller is needed which can operate to produce a virtual control effect which is equivalent in ENERGY SAVINGS with that of my prior invention's unique multiple RUN winding embodiment. I therefore conceived a controller that produces such improvement, but requires no change in the older motor's design: e.g., it operates well with merely a single RUN winding arrangement in the motor.
The need for my current invention is to SAVE ENERGY in pre-existing motor applications, particularly such as found in air conditioners and similar equipment.
Manufacturer's of new equipment may also benefit from the ENERGY SAVING contribution of my invention without having to re-engineer the electric motor which may already be part of a proven product design, or consist of considerable inventory.